1. Field of the Invention
This invention relates generally to a technique for controlling the temperature of a fuel cell stack in a fuel cell system and, more particularly, to a technique for controlling the temperature of a fuel cell stack in a fuel cell system by using a lumped parameter non-linear thermal model of the stack to anticipate the temperature of the cooling fluid out of the stack and controlling a pump in response thereto.
2. Discussion of the Related Art
Hydrogen is a very attractive fuel because it is clean and can be used to efficiently produce electricity in a fuel cell. The automotive industry expends significant resources in the development of hydrogen fuel cells as a source of power for vehicles. Such vehicles would be more efficient and generate fewer emissions than today's vehicles employing internal combustion engines.
A hydrogen fuel cell is an electro-chemical device that includes an anode and a cathode with an electrolyte therebetween. The anode receives hydrogen gas and the cathode receives oxygen or air. The hydrogen gas is dissociated in the anode to generate free hydrogen protons and electrons. The hydrogen protons pass through the electrolyte to the cathode. The hydrogen protons react with the oxygen and the electrons in the cathode to generate water. The electrons from the anode cannot pass through the electrolyte, and thus are directed through a load to perform work before being sent to the cathode. The work acts to operate the vehicle.
Proton exchange membrane fuel cells (PEMFC) are a popular fuel cell for vehicles. A PEMFC generally includes a solid polymer electrolyte proton conducting membrane, such as a perfluorosulfonic acid membrane. The anode and cathode typically include finely divided catalytic particles, usually platinum (Pt), supported on carbon particles and mixed with an ionomer. The catalytic mixture is deposited on opposing sides of the membrane. The combination of the anode catalytic mixture, the cathode catalytic mixture and membrane define a membrane electrode assembly (MEA). MEAs are relatively expensive to manufacture and require certain conditions for effective operation. These conditions include proper water management and humidification, and control of catalyst poisoning constituents, such as carbon monoxide (CO).
Many fuel cells are typically combined in a fuel cell stack to generate the desired power. The fuel cell stack receives a cathode input gas, typically a flow of air forced through the stack by a compressor. Not all of the oxygen in the air is consumed by the stack and some of the air is output as a cathode exhaust gas that may include water as a stack by-product. The fuel cell stack also receives an anode hydrogen input gas that flows into the anode side of the stack.
The fuel cell stack includes a series of bipolar plates positioned between the several MEAs in the stack. The bipolar plates include an anode side and a cathode side for adjacent fuel cells in the stack. Anode gas flow channels are provided on the anode side of the bipolar plates that allow the anode gas to flow to the MEA. Cathode gas flow channels are provided on the cathode side of the bipolar plates that allow the cathode gas to flow to the MEA. The bipolar plates are made of a conductive material, such as stainless steel, so that they conduct the electricity generated by the fuel cells out of the stack. The bipolar plates also include flow channels through which a cooling fluid flows.
It is necessary that a fuel cell operate at an optimum relative humidity and temperature to provide efficient stack operation and durability. The temperature provides the relative humidity within the fuel cells in the stack for a particular stack pressure. Excessive stack temperature above the optimum temperature may damage fuel cell components, reducing the lifetime of the fuel cells. Also, stack temperatures below the optimum temperature reduces the stack performance.
Fuel cell systems employ thermal sub-systems that control the temperature within the fuel cell stack. Particularly, a cooling fluid is pumped through the cooling channels in the bipolar plates in the stack. FIG. 1 is a schematic plan view of a fuel cell system 10 including a thermal sub-system for providing cooling fluid to a fuel cell stack 12. The cooling fluid that flows through the stack 12 flows through a coolant loop 14 outside of the stack 12 where it either provides heat to the stack 12 during start-up or removes heat from the stack 12 during fuel cell operation to maintain the stack 12 at a desirable operating temperature, such as 60° C.-80° C. An input temperature sensor 16 measures the temperature of the cooling fluid in the loop 14 as it enters the stack 12 and an output temperature sensor 18 measures the temperature of the cooling fluid in the loop 14 as it exits the stack 12.
A pump 20 pumps the cooling fluid through the coolant loop 14, and a radiator 22 cools the cooling fluid in the loop 14 outside of the stack 12. A fan 24 forces ambient air through the radiator 22 to cool the cooling fluid as it travels through the radiator 22. A controller 28 controls the speed of the pump 20 and the speed of the fan 24 depending on the temperature signals from the temperature sensors 16 and 18, the power output of the stack 12 and other factors.
Because the membranes in the fuel cell stack 12 are very sensitive to damage and require a strict relative humidity control for efficient stack operation, it is important to precisely control the internal temperature of the fuel cell stack 12. The current temperature control systems monitor the output temperature of the stack 12, and as the temperature of the cooling fluid from the stack 12 changes, the controller 28 increases or decreases the speed of the pump 20 and the speed of the fan 24 to provide more or less cooling. However, if the temperature of the stack 12 has already increased or decreased before the pump 20 responds, the relative humidity of the membranes has changed. It would be desirable to anticipate an increase or decrease in the temperature of the stack 12, and change the flow rate of the cooling fluid before the temperature of the stack 12 significantly changes.